Free tool

Sleep calculator

Tell us what time you want to get up (or what time you go to bed) and we suggest recommended times based on 90-minute sleep cycles, allowing about 15 minutes to fall asleep.

Calculations run in your browser: we do not send or store the data you enter.

How this calculator works

Sleep is organised in cycles of about 90 minutes. If you wake up at the end of a cycle, you usually feel less groggy than if the alarm goes off in the middle of deep sleep. The calculator counts backwards (or forwards) in 90-minute blocks and adds the time it takes you to fall asleep.

To wake up at a specific time: we subtract 4, 5 or 6 cycles plus your time to fall asleep.
If you go to bed at a specific time: we add your time to fall asleep and then 4, 5 or 6 cycles.

How many cycles you need

Cycles	Hours of sleep	Who it usually suits
4	6 hours	Occasional minimum; not recommended on a continuing basis
5	7.5 hours	Adults in general
6	9 hours	People who need more rest or are recovering from lack of sleep

Limits of this method

90-minute cycles are an average: in reality they vary between people and across the night, and can range from about 80 to about 110 minutes. The calculator is a practical guide, not a measurement. If you persistently wake up tired, snore heavily or have breathing pauses, you should consult a healthcare professional.

Tips to make the calculated time work

Keep the same wake-up time every day, including at weekends.
Avoid caffeine from mid-afternoon and heavy dinners.
Dim the lights and cut back on screens in the hour before bed.
Aim for a dark, quiet and cool room.

To see how much of your life you spend sleeping, read the article How much of your life do you spend sleeping?

Frequently asked questions

Why is time to fall asleep counted?

Because nobody falls asleep the instant the light goes off. On average it takes about 10 to 20 minutes; that is why the calculator uses 15 by default and you can change it.

Is it better to sleep 6 hours in complete cycles than 7 in incomplete ones?

Not necessarily. Complete cycles help you wake up better, but total sleep matters too. If you can, aim for 5 or 6 cycles.

Does this work for naps?

Naps work differently: they are usually kept short, 20 to 30 minutes, to avoid entering deep sleep.

Is this medical advice?

No. It is an indicative tool and does not replace the assessment of a healthcare professional.